1. Field of the Invention
This invention relates to a digital signal processing circuit, and more particularly to a digital signal processing circuit having a pseudorandom noise generative function for use of a digital signal processing.
2. Description of the Related Art
It is necessary to use pseudorandom noise for a predetermined process in a signal processing unit processing a time series digital data, in addition to processing a normal input data. In the field of audio units, there are applications such as the Dolby pro-logic which must have the pseudorandom noise generative function for setting up an output level of peripheral equipments such as speakers.
A linear feedback shift register has been well known as the pseudorandom noise generative unit and is disclosed in the Japanese laid-open publications Nos. 63-82014 and 63-250210. The linear feedback shift register comprises a circuit in which flip-flops are serially connected to compose a shift register and some outputs of the flip-flops are fed back to an input of the shift register through an exclusive OR gate. An n-bits linear feedback shift register generates an M series pulse, the pseudorandom noise, having a cycle of N (.ltoreq.2.degree.-1).
FIG. 1 shows an example of a 4 bit linear feedback shift register. When an initial state of the linear feedback shift register is (Q.sub.1 Q.sub.2 Q.sub.3 Q.sub.4)=(1000), the outputs of each flip-flop are changed as shown in FIG. 1 and the outputs of them also return to the initial state when the 15th clock arrives in each flip-flop. In this case, the output patterns of each flip-flop are pseudorandom patterns having a maximum length (N=15) which may be generated by the 4-bits linear feedback shift register.
There are two types of the conventional digital signal processing circuit with the pseudorandom noise generative function. One is provided with the linear feedback shift register for generation of the pseudorandom noise as the another circuit and the other one uses its arithmetic and logic circuit (ALU) and the shift register to generate the pseudorandom noise in part of an arithmetic processing without using the linear feedback shift register.
The conventional digital signal processing circuits with the pseudorandom noise generative function are, however, engaged with a disadvantage as follows. In the former, it is necessary to have hardware resources such as the linear feedback shift register and the like. Thus, a scale of the circuit is made larger because of an addition of the hardware resources. On the contrary, the hardware resources added to the circuit are not used in the normal digital signal processing mode which is not necessary to use the pseudorandom noise at all. In addition, a data input port of the digital signal processing circuit must be changed to connect with an output of the linear feedback shift register for fetching the pseudorandom noise data to be processed from the linear feedback shift register, for which it is required to change a processing sequence of the digital signal processing circuit.
In the latter, the number of the steps for processing the data is increased since the pseudorandom noise generative process is added to the normal input data process. Thus, an ability of the processing unit is remarkably decreased when the time series digital data such as the audio data is processed by a real-time processing.